The citral compound of formula I designates citral as all of its isomers, i.e. neral and geranial as well as mixtures of these compounds. The Z-form or the E-form of the compound of formula II may be used as starting materials.
Also, mixtures of these E and Z forms can be used as the starting materials of formula II. The ratio of E/Z has little influence on the course of the reaction or on the yield. These starting materials of formula II are known or can be prepared according to methods known per se.
Suitably, R.sup.1 and R.sup.2 are ethyl or propyl or R.sup.1 and R.sup.2 together with the N-atom represent piperidino, pyrrolidino or perhydroazepin.
R.sup.1 and R.sup.2 preferably represent ethyl.
The preferred C.sub.2-6 -alkanecarboxylic acid is acetic acid. Anhydrides are formed also with C.sub.2-6 alkane carboxylic acids.
The reaction in accordance with the invention can be carried out under acidic or basic conditions, namely either with the aid of, for example, one of the aforementioned acids as the solvent or, for example, with the addition of a liquid tertiary amine (e.g. triethylamine, diethylmethylamine, pyridine, etc.).
Conveniently, the compound of formula I is placed in one of the aforementioned acids or one of the foregoing amines and the acylating agent is slowly added thereto at a temperature of -20.degree. to 20.degree. C., especially 0.degree. to 20.degree. C. However, temperature is not critical, and any suitable temperature can be used to carry out the reaction of this invention. Furthermore, the mole ratio of the compound of formula II to the acylating agent is also not critical and any convenient mole ratio can be used in carrying out this reaction. However, the molar ratio of the compound of formula II to acylating agent that is preferred is 1:1-5, with the ratio 1:3-5 being especially preferred.
The reaction in accordance with the invention is thereupon carried out, conveniently at a temperature between 0.degree. and 100.degree. C., preferably between 20.degree. and 60.degree.. The course of the reaction can be followed, for example, by gas chromatography.
For the working-up, an extraction agent (e.g. an ether) and water are added to the reaction mixture. Furthermore, the presence of a buffer substance such as a bicarbonate and an alcohol (e.g. i-propanol) has been found to be advantageous.